The borehole of producing oil and gas wells is typically lined from top to bottom with steel casing anchored by a sheath of cement that is securely and circumferentially bonded to both the casing and the wall of the wellbore. Oftentimes production wells are drilled at extreme depths with the drilled wellbore deviating substantially from the vertical. There is, therefore, the need for a reliable method of locating the wellbore of such production wells, especially when a new well is being drilled in the vicinity of the producing well to prevent the drilling into such producing well.
In one conventional drilling practice, the direction and position of the production wellbore was determined at the time of drilling by running hole direction surveys and processing the directional data for wellbore position. The directional measurements are commonly made using a magnetic compass and inclinometer. The standard deviation of these directional measurements is not accurately known, but is probably greater than the resolution of the instruments. There are other random errors due to instrument friction and mounting instability, tool alignment in the wellbore, gravity pertubations, magnetic variation uncertainty, magnetic disturbance of the drill pipe, etc. In addition to these random errors, there are bias errors. The largest bias error is probably in the compass reading caused mostly by poor calibration and the drillpipe magnetic disturbance. The total directional bias error can range up to a few degrees or even more.
Another method that has been used in such well location efforts is by searching with a magnetometer in the well being drilled for the magnetic anomaly created by the well casing of the producing well, as taught in U.S. Pat. No. 4,458,767 to Hoehn. The natural magnetization of the well casing due to the earth's magnetic field produces an anomaly in the total magnetic field which may be detected with a proton magnetometer at distances up to a few hundred feet. Also, a well casing may be magnetized by means of an internal magnetizer being advanced through the well casing to create a magnetic anomaly along the well casing, as taught in U.S. Pat. No. 4,465,140 to Hoehn.
In U.S. Pat. No. 4,372,398 to Kuckes, there is described a system wherein the well being drilled contains a device for magnetizing a different well through a current injection by way of the formation between the well being drilled and such different well.